1. Field of the Invention
The present invention relates to a FSK (Frequency Shift Keying) receiver device for use in radio communication using the frequency shift keying.
2. Description of the Related Art
A low-cost FSK communication system is often used in narrow-band and near-field radio communication. In particular, Low-IF type FSK receiver device is widely used in recent years because the Low-IF type FSK receiver device can incorporate a band pass filter thereinto to thereby reduce cost. Typically, in the FSK receiver devices, a demodulation scheme is adopted in which an input signal (radio signal) is amplitude-limited by a limiter amplifier and the like and then a demodulation signal is obtained by quadrature detection using an external phase shifter. As a low cost scheme cheaper than the above mentioned one, a demodulation scheme is known in which a demodulation signal is obtained by performing differentiation and multiplication processes for complex I/Q baseband signals (output signals from I/Q down-converter) (see, for example, “A Single-Chip VHF and UHF Receiver for Radio Paging”, by John F. Wilson, Richard Youell, Tony H. Richards, Gwilym Luff, and Ralf Pilaski, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 26, NO. 12, Dec. 1991).
Further, for a communication scheme using digital frequency modulations, a delay detection scheme is known which can efficiently detect and demodulate a signal transmitted in the state that a frequency shift is lowered in order to use the frequency spectrum effectively (for example, see Japanese Unexamined Patent Application Publication NO. Sho 61-99447).
In the demodulation scheme in which a demodulation signal is obtained by quadrature detection using an external phase shifter, the external phase shifter costs a lot as compared to others. For this reason, this scheme is not suitable for the FSK receiver device of which cost reduction is strongly demanded. Also, in such a scheme, the external phase shifter may need to be adjusted at the time of shipping the FSK receiver device, which results in a disadvantage in the scheme mentioned above.
The demodulation scheme, in which a demodulation signal is obtained by performing differentiation and multiplication processes for complex I/Q baseband signals, is suitable for a case where the modulation index is large as in a conventional pager. But, when the modulation index is small (for example, ECHONET: modulation index is 1.75 when frequency shift is 2.1 kMHz and data rate is 2.4 kbps), the variation in the demodulation signal is large. In other words, the duty of the demodulation signal deviates, resulting in large jitter. For this reason, it is difficult for a baseband IC in a later stage to be synchronized with the demodulation signal.